1. Field of the Invention
The present invention relates to communications. More particularly, the invention concerns a method and apparatus for acquiring a base station""s transmission signal in a multi-carrier spread spectrum communication network.
2. Description of the Background Art
The use of code division multiple access (CDMA) modulation is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. However, the spread spectrum modulation technique of CDMA has significant advantages over these modulation techniques for multiple access communication systems. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, issued Feb. 13, 1990, entitled xe2x80x9cSPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS,xe2x80x9d assigned to the assignee of the present invention and incorporated by reference herein.
Path diversity is obtained in CDMA systems by providing multiple signal paths through simultaneous links between a remote station and two or more cell-sites. Furthermore, path diversity may be obtained by exploiting this multi-path environment through spread spectrum processing, thereby allowing signals on the same frequency arriving with different propagation delays to be received and processed separately. Examples of path diversity are illustrated in U.S. Pat. No. 5,101,501, issued Mar. 31,1992, entitled xe2x80x9cSOFT HANDOFF IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d and U.S. Pat. No. 5,109,390, issued Apr. 28, 1992, entitled xe2x80x9cDIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d both assigned to the assignee of the present invention and incorporated by reference herein. Further, by CDMA""s inherent nature of being a wideband signal, it offers a form of frequency diversity by spreading the signal energy over a wide bandwidth. Therefore, frequency selective fading affects only a small part of the CDMA signal bandwidth.
Fading can have deleterious effects on signals but can be controlled to a certain extent by controlling transmitter power. A system for cell-site and remote station power control is disclosed in U.S. Pat. No. 5,056,109, issued Oct. 8, 1991, entitled xe2x80x9cMETHOD AND APPARATUS FOR CONTROLLING TRANSMISSION POWER IN A CDMA CELLULAR MOBILE, TELEPHONE SYSTEM,xe2x80x9d Ser. No. 07/433,031, filed Nov. 7, 1989, now U.S. Pat. No. 5,056,109 assigned to the assignee of the present invention and incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,xe2x80x9d assigned to the assignee of the present invention and also incorporated by reference herein. In general, the fading effects for related carriers in a multi-carrier communication network are approximately the same.
In the patents mentioned above, a pilot signal is used to acquire a base station""s transmission signal. Acquisition means that the remote station detects and verifies the existence of such a signal. Detection means that a signal is present and the remote station detects it. The remote station is able to lock onto the pilot signal more easily than other signals because it is unmodulated by data and is usually transmitted at a higher power level than other signals. A pilot signal is an unmodulated, direct-sequence spread spectrum signal transmitted by a base station or remote station. Generally, a pilot signal provides a phase reference for coherent demodulation of a traffic channel, but may also be used for signal strength comparisons between base stations. Using a pilot signal enables a station to acquire a carrier provided within the traffic channel from a local base station communication network in a timely manner. The remote station gets synchronization information and relative signal power information regarding the carrier from the received pilot signal. The remote station may receive signals from multiple base stations in the network and more than one multipath from each base station. Multipath refers to the possible multiple signals arriving at a receiver antenna at different times. Signals that are in phase will add, and signals that are out of phase will cancel one another.
Scanning across an entire code domain in a uniform manner introduces intolerable delays, so remote stations usually scan pilot signals selectively. For example, a remote station may search around pilot signal offsets in active, candidate, and remaining lists. Offset refers to the different time offset for a signal. A search window, typically between 4 and 130 chips, is specified for each pilot signal offset. The remote station detects a signal by looking at the pilot signal energy on the corresponding time offset.
Further, a traffic channel between a remote station and a base station may comprise one or multiple carriers. A carrier is the underlying frequency or frequencies that are used to carry information. They are modulated through one or more modulation techniques to impose information on a signal. For example, a multi-carrier forward traffic channel, also referred to herein as a forward link (FL), may define a mode of operation used with a spreading rate S where S greater than 1, and that uses X adjacent direct-spread radio frequency (RF) carriers. Interleaved data may be demultiplexed onto each of the X adjacent carriers. For example, FIG. 1 shows a diagram used to illustrate three (X=3) frequency bands 102, 104, and 106 of width 1.25 MHz, common to some CDMA communication networks. From each frequency band, a carrier is selected to form a multi-carrier (MC) forward link 108 that facilitates communication between one or more base stations and a remote station.
Regardless of whether a traffic channel provides a single carrier or multi-carriers, a search of pilot signals to find a phase reference for coherent demodulation of a candidate traffic channel is currently conducted the same way. The mobile station generates a local version of a pilot signal pseudonoise (PN) sequence with a guessed time offset (called a hypothesis), correlates this local PN sequence with the received signal for a number (N) of PN chips, and noncoherently combines the energy of M such coherently integrated energies. The result is compared to a first threshold (T1). This is the first dwell in a multiple-dwell process. If the accumulated energy is below T1, the mobile station considers that there is no signal present on that time hypothesis and moves to the next hypothesis. This is also referred to as an early dump. Otherwise, the mobile station performs verification on that time hypothesis by using a similar procedure, typically with a different (usually larger) value of N and M. If the accumulated energy obtained in the verification is below a threshold of T2, then the time hypothesis is discarded. Otherwise, the mobile station performs further verification. If the accumulated energy surpasses the threshold in the final verification, then acquisition is declared on that time hypothesis and a demodulator is assigned to demodulate the signal at that time hypothesis. FIG. 2 illustrates such a method 200.
The method 220 starts in task 202 and yields the results discussed below with reference to FIG. 3. A window is searched or xe2x80x9csweptxe2x80x9d in task 204. If a hypothesis contained in the window does not exceed a first detection threshold (FTV) as shown in task 206, and if all hypotheses in the window have not been searched in task 208 for the specified parameters, then the search is repeated using a larger window in task 210. If all hypotheses for a selected set of parameters have been searched in task 208, then hypotheses for a next set of selected parameters are used in task 212 and the sweep is repeated in task 204.
However, if there are points on the calculated energy curve which do exceed the UTV, then the method 200 proceeds to a validation phase shown as task 214. In task 214, the same large window is swept again, but this time the calculated energy is compared against a second threshold value, or validation threshold (STV), in task 216. If the maximum energy detected is not greater then the STV, the method 200 returns to task 208 and a next large window is swept. If the detected energy exceeds the STV, and if the result has been validated for N consecutive windows of data, then a pilot has been acquired and the method 200 ends in task 220. If fewer than N validation tests have been conducted, then the method 200 returns to task 214 and the large window is swept again. The value of N may be set to any value, such as 20, that is desired to assure within a reasonable certainty that a false acquisition has not occurred.
An example of using a fixed window size follows. FIG. 3 illustrates a graph of energy values versus the chip time hypothesis. In the illustrated embodiment, the window is 56 chips in width. However, a 64 chip width or other sized window may be used. The window illustrates the use of a two level threshold test. The thresholds denoted are a two detection thresholds and a validation threshold. When a peak is detected, the searcher controller concentrates on that peak and tests hypotheses close to the hypothesis that gave rise to the detected peak. Further discussion of fixed window search methods may be found in U.S. Pat. No. 5,805,648, issued Sep. 8, 1998, entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM,xe2x80x9d and U.S. Ser. No. 08/509,721, filed Jul. 31, 1995, now U.S. Pat. No. 5,805,648 entitled xe2x80x9cMETHOD AND APPARATUS FOR PERFORMING SEARCH ACQUISITION IN A CDMA COMMUNICATION SYSTEM,xe2x80x9d both of which are assigned to the assignee of the present invention. In a MC system, this process is repeated for a pilot signal associated with each carrier.
In an ideal system where the time for setting up the search hardware is zero, a method providing one hypothesis for searching a carrier would be fine. Realistically, because it takes time to set up the hardware to conduct a search, xe2x80x9cwindowsxe2x80x9d of search hypotheses are usually used. Generally speaking, the longer the time required for setting up the hardware, the larger the size of the window used. In complex systems, a searcher is required to search a window of many hypotheses. Upon finding a candidate synchronized sequence, the searcher will repeat the search using a smaller window to verify the synchronization. For networks using a single carrier, this method may be acceptable. But in a MC network, this methodical approach results in a long time delay in acquiring all of the carriers.
What is needed is an invention that provides a method and apparatus shortening the time required to identify and access a traffic channel""s carriers as used in a MC communications network. The invention should also reduce the resource requirements for performing such a routine.